I. Field of the Invention The present invention relates generally to hydraulic fluid reservoirs. More particularly, the present invention relates to a system for increasing the cooling capacity of hydraulic fluid reservoirs by building into a reservoir an air-to-fluid heat exchanger.
II. Description of the Relevant Art
Historically, the hydraulic tank has been used to store fluid, remove heat generated in the hydraulic system and provide de-aerating time for the hydraulic fluid.
If the fluid is not cooled properly, it can break down and/or cause other components in the hydraulic system to fail. Furthermore, if the fluid is not de-aerated properly the hydraulic pump is susceptible to cavitation damage and sure failure.
The hydraulic reservoir has been made large so that the air or vapor bubbles in the fluid returning to the reservoir would have time to float to the fluid surface and break before the aerated fluid could be recirculated by the pump. Additionally, the large reservoir walls were used to radiate the heat in the returning fluid to the atmosphere. However, the heat dissipation rate, or fluid cooling rate, is very slow because: first, the heat in the fluid is first dissipated by transferring it to a relatively large volume of fluid in the reservoir; second, the heat is then transferred to the reservoir walls by a relatively slow mixing or circulation rate that occurs within the reservoir; third, the dissipated heat is then transferred through the reservoir walls; and fourth, the outer wall surface transfers the heat to the atmosphere by radiation.
Because the current state-of-the-art reservoir is so lacking in its ability to remove heat from the fluid, the reservoir has limited the pressure of many industrial hydraulic systems for over 30 years to 500 psi. For example, about 90% of the U.S. automobile industry limits their machine tool and automation hydraulic systems to 500 psi. Systems operating above 500 psi generate more heat than the reservoir can dissipate. As a result, the reservoirs overheat and the hydraulic equipment fails unless an auxiliary heat exchanger is added to the power unit. When auxiliary heat exchangers were added to the system the same large reservoir was required because of the time needed for the fluid to de-aerate itself.
The floor space required for these large reservoirs and associated hydraulic power units is two or three times larger than the pump/electric motor floor space requirement. Such elements are normally bolted to the floor. Accordingly, if a power unit stopped for any reason, the machine tool or system was stopped until the power unit was fixed.
Millions of gallons of oil are imported each year to fill these reservoirs and millions of gallons are disposed of each year. Thus, the requirements of the large reservoir also has a negative effect on our nation's balance of payment. Furthermore, disposition of the oil damages the environment.